$$a^4-6a^2+1 = 0$$
Answer
$$ \begin{matrix}a_1 = 0.41421 & a_2 = -0.41421 & a_3 = -2.41421 \\[1 em] a_4 = 2.41421 & \\[1 em] \end{matrix} $$
Explanation
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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